Run crontab in docker. Django

Sorry for my english. I created docker file. For cron task i user this library, In my local test all work, but now i try use docker and try run cron there.But now i have error

no crontab for root

My docker file:

FROM python:3.6
WORKDIR /usr/src/app
COPY requirements.txt ./
RUN apt-get update && apt-get install -y cron
RUN pip install -r requirements.txt
COPY . .
EXPOSE 8000
CMD python manage.py runserver 0.0.0.0:8000
RUN python manage.py crontab add
RUN python manage.py crontab show

My requirements

Django==1.11.4
django-filter==1.0.4
djangorestframework
Pillow
django-crontab